Personally, I prefer an octo. More options (either one of you can use it) and you've got a longer hose which makes it easier to deal with the OOA person.

2. How often have you had to donate air?

Not counting teaching scenarios . . . never.

3. Do people think running out of air/having to share air is a serious situation?

Absent equipment failure (a rarity) running out of the air is the stupidest thing you can do while diving. It simply shows you're not paying attenton to things. The second-stupidest thing (IMHO) is being nonchalant about it. Personally, I wouldn't dive with that person again.